J-Pop on the iTunes Store. There's not a great deal of it, and it can be hard to find (or I just suck at using the store). These are the artists/albums I've found on there (not intended to be an even slightly comprehensive list, I've only looked for artists I like ):

@Raylax's J-Pop requestWhile I don't quite know if I'd call it "pop" (it is, but electronic influenced and exploding with energetic vocalists), iTunes has one of my favorite bands: Polysics. At a glance, they have four complete albums up there. One appears to have a digital booklet as well.